Current-induced magnetization switching of thermally quasistable magnetic nanoislands is demonstrated using a spin-polarized scanning tunneling microscope. The magnetization of an individual Fe nanoisland consisting of about 40 atoms on a W(110) surface is reversibly switched between two quasistable states by the application of spin-polarized tunnel current pulses without an applied magnetic field. The pulse length is shown to be crucial for a high switching efficiency. Sweeping the tunnel current from the nanoampere to the microampere regime allows for the determination of critical switching currents. (C) 2010 American Institute of Physics. {[}doi: 10.1063/1.3354023]